Numerous processes are known for decomposing polyurethane foam waste by hydrolytic degradation into a mixture of the polyol used as a starting material and the diamine from which the diisocyanate used for the production of the foam was obtained (see e.g., German Offenlegungsschrift No. 24 42 387). Before this hydrolysate mixture is used again in a production process, it must be separated into the polyol and the diamine component.
A separation process, if it is to be commercially useful, should operate economically, should not require high investment costs and should deliver the isolated raw materials in a form in which they can, as far as possible, be used directly, i.e. without elaborate purification. In the case of the rather delicate compositions for polyurethane soft foams, which are very sensitive to any interfering influences such as impurities, this applies particularly to the quality of the polyether polyols which are recovered from the hydrolysates to be used again in the foaming process.
Separation of the hydrolysate mixture has hitherto been carried out mainly by distillation and liquid extraction processes in various modifications. The diamine component is recovered in a high yield and in a high degree of purity from the distillation process but the polyether obtained as the distillation residue is unsuitable for direct reuse in foaming in the form in which it is obtained (Environmental Science and Technology, Volume 8, No. 2, February 1974, page 138).
In the liquid extraction process as described, for example, in German Auslegeschrift No. 2,207,379, the hydrolysate is taken up in toluene or some other organic solvent and the diamine is extracted using dilute aqueous hydrochloric acid. The limited solubility of the diamine hydrochloride necessitates the use of a considerable volume of dilute acid, which must subsequently be removed from the extract by distillation. Moreover, the stabilizers and emulsifiers which were used in the production of the foam make separation of the organic and aqueous phases very difficult.
U.S. Pat. No. 4,035,314 describes the recovery of polyurethane raw materials by the hydrolysis of plastic materials obtained from automobile scrap. In this process, when the polyurethane hydrolysate which consists mainly of polyether and diamine has been separated from the unhydrolysable synthetic materials by means of an organic solvent, gaseous hydrogen chloride is introduced into the solution of hydrolysate until no more amine hydrochloride precipitates. After removal of the amine hydrochloride by filtration and removal of the solvent and excess hydrogen chloride by distillation, a regenerated polyether is obtained. This is reacted with tolylene diisocyanate to form a prepolymer which, when mixed with pure prepolymer, can be hardened with 4,4'-methylene dianiline to produce an elastomer. Regenerated polyethers used in this way are, however, unusable for the production of foams because they contain large residues of amines even when a clear excess of hydrogen chloride has been used.
It has now surprisingly been found that a regenerated polyether which is virtually free from amine and which can be foamed satisfactorily is obtainable by fractionating the precipitation of amine hydrochloride, i.e. by carrying it out in several stages, and separating the precipitated amine salt from the solution of hydrolysate at each stage before a fresh supply of hydrogen chloride is introduced.